The present invention relates to a film-forming composition and to a process for its production, and particularly relates to a composition for formation of hydrophilic films.
Coating solutions have conventionally been prepared to form various different coatings for protection of surfaces of a variety of articles. The coatings produced by these coating solutions include hydrophilic coatings and water-repellent coatings. Both hydrophilic coatings and water-repellent (hydrophobic) coatings are effective for surface protection and antifouling effects. Water-repellent resin coating films and the like have been widely used in the prior art. However, hydrophilic coatings are effective in avoiding blockage of visibility on window glass or mirrors, or in preventing adhesion of oily contaminants that are abundant in metropolitan regions. Formation of hydrophilic coatings has therefore become more desired in recent years.
Some of these coatings include photocatalysts such as titanium oxides. When irradiated with light, the photocatalysts in photocatalyst-containing coatings decompose organic substances. Such coatings, therefore, are often used for coating of a variety of familiar articles for the purpose of providing the articles with antibacterial properties, preventing adhesion of organic substances on air conditioner evaporators, and the like. A photocatalyst-containing coating of this type will normally exhibit hydrophobic properties when not irradiated with light, due to the photocatalyst itself or to alkyl groups present in the coating with the photocatalyst. When the coating is irradiated with light, however, the photocatalyst effect causes adsorption of hydroxyl groups onto the surface from moisture in the air, causing the coating to exhibit hydrophilic properties.
Furthermore, such hydrophilic coatings have low adhesive force on substrate surfaces and low surface hardness. Consequently, rubbing causes the coatings to easily peel from the substrate surfaces on which they are formed. Attempts to increase the peeling strength of the coatings usually require burning and other such steps, and therefore it has been difficult to form coatings that can be coated with simple procedures such as spraying and drying, and that exhibit strong adhesive force and high hardness.
Some photocatalyst-containing coatings exhibit hydrophilic properties when irradiated with light. However, as time passes in the absence of light irradiation, the surface hydrophilic groups are lost and hydrophobic properties are exhibited that result in adhesion of water droplets during nighttime, thereby promoting adhesion of dirt and making it impossible to adequately exhibit the antifouling effect of the photocatalyst, once such dirt has adhered.
Even coatings with hydrophilic groups on the surface usually have contact angles of about 50xc2x0 for water and therefore cannot be said to have sufficient wettability, and since water droplets readily form when water adheres to the surface, a problem has existed in that it has not been possible to exhibit an adequate antifouling effect by hydrophilic groups.
It is an object of the present invention to overcome these problems by providing a film-forming composition that can form strong coatings that exhibit hydrophilic properties. It is another object to provide coatings with sufficient hydrophilic properties or electronic properties to prevent adhesion of dirt or dust.
In order to solve the aforementioned problem, the film-forming composition of the invention comprises a liquid containing a binder and a hydrotalcite-type compound represented by [M2+1xe2x88x92xM3+x(OH)2]x+[Anxe2x88x92x/nmH2O]xxe2x88x92 (where M2+ is a bivalent metal ion, M3+ is a trivalent metal ion, Anxe2x88x92 is an anion of valency n and 0 less than xxe2x89xa60.33). The composition may be coated and dried to form a coating that exhibits hydrophilic properties. The coating is resistant to smearing, and can be provided with sufficient hardness to withstand a normal living environment.
The composition preferably contains transition metal oxide particles, preferably titanium oxide particles. Inclusion of transition metal oxide particles makes amount of adherent dirt or dust to coatings decrease extremely and hardness of coatings increase. Furthermore, inclusion of titanium oxide particles has also been confirmed to improve the wettability of formed coatings, impart antibacterial properties and cause oxidation reaction on SOx and NOx.
The mean particle size of the hydrotalcite-type compound is 3-500 xcexcm, and it also preferably includes fine particles of a stable inorganic compound, having a mean particle size of 1 xcexcm or smaller. This will increase the smoothness of the coating surface and reduce the contact angle for water and the like, thus providing improved wettability and reducing the degree of fouling of the coating surface. The mean particle size of the inorganic compound is preferably 0.001-0.5 xcexcm, and even more preferably 0.01-0.1 xcexcm. If the mean particle size exceeds 1 xcexcm, the effect of improved wettability will be reduced, while the catalyst performance per unit weight is also thought to be inferior. On the other hand, it is very difficult to form fine particles with a mean particle size of smaller than 0.001 xcexcm, and this will increase production cost and reduce the effect of improved smoothness of the coating surface.
The inorganic compound is preferably a transition metal oxide. Transition metal oxides can be easily formed into fine particles of 1 xcexcm or smaller by chemical synthesis or the like. And dirt and dust hardly adheres to the coatings by inclusion of transition metal oxide particles to the film-forming composition (coating liquid or solution) while also increasing the coating hardness; by using substances with catalytic functions that decompose organic matter (such as titanium oxides) as the transition metal oxide it is possible to improve the antifouling property of the coating, while using water-absorbing substances (such as magnesium oxides and aluminum oxides) can improve the hydrophilic property and wettability.
Transition metal oxide particles in the coating film are covered with the hydrotalcite-type compound, therefore surface region of the coating film becomes N-type semiconductor and it becomes hardly charged with electricity. Then, anti-pollution property is obtained, i.e. it is preventable from adhesion of dirt or dust to the coating film. The coating films formed by using the film-forming composition (coating liquid or solution) of this invention have electronic properties of the semiconductor which has surface electric resistivity (specific resistance) of 108-1013  xcexa9 cm. For example, when dust charged with electricity is applied on the coating film, dust become unbalance in electricity and easily drop out of the surface of the coating film without adhering to the surface thereof.
Here, it is preferred for the transition metal oxide to be a substance that is stable in normal environments, and typical examples that may be used include titanium oxides (TiO, Ti2O3, TiO2), manganese oxides (MnO, Mn2O3, MnO2, Mn3O4, Mn2O7) andiron oxides (FeO, Fe2O3, Fe3O4). Other stable transition metal oxides that exist include oxides of Sc, V, Cr, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Ef, Ta, W, Re, Os, Ir, Pt, Au and Hg. These can provide the coating itself with antibacterial properties and absorption or catalytic action on NOx and SOx.
It is preferred for at least a portion of the transition metal oxide to be titanium oxide. The titanium oxide particles may have any desired shape. The mean particle size of the titanium oxide is 1 xcexcm or smaller, preferably 0.001-0.5 xcexcm, and more preferably 0.01-0.1 xcexcm.
The hydrotalcite-type compound preferably contains Mg2+ as M2+, Al3+ as M3+ and CO33xe2x88x92 as Anxe2x88x92. Magnesium-aluminum-hydroxide-carbonate hydrates are substances with high stability among hydrotalcite-type compounds, and are easily obtainable. Here, x=0.33, n=2 and m=4 is most preferred.
The binder is preferably colloidal silica. The binder is preferably an inorganic binder, and colloidal silica is most preferred from the viewpoint of composition stability, coating adhesion, transparency of the coating film, etc.
For the means described above, the liquid (the film-forming composition) preferably contains water, solvents comprising a mixture of water and an alcohol, or other organic solvents. Dilution with a solvent comprising water or alcohol can be used to adjust the transparency of the coating and the pH, hardness and volatility of the composition.
The preferred process for production of the aforementioned film-forming composition is one wherein a hydrotalcite-type compound represented by [M2+1xe2x88x92xM3+x(OH)2]x+[Anxe2x88x92x/nmH2O]xxe2x88x92 (where M2+ is a bivalent metal ion, M3+ is a trivalent metal ion, Anxe2x88x92 is an anion of valency n and 0  less than xxe2x89xa60.33) is dispersed in water under acidic conditions and then mixed with the binder. Dispersing the hydrotalcite-type compound in the solvent including water under acidic conditions and mixing it with the binder at a same time or one after another will facilitate dispersion of the hydrotalcite-type compound, yielding a film-forming composition that avoids aggregation of the hydrotalcite-type compound while allowing formation of a highly transparent coating.
Here, the inorganic compound (transition metal oxide) particles may be added the hydrotalcite-type compounds under acidic conditions. In this case, the inorganic compound particles may be mixed before, at a same time, and after the hydrotalcite-type compound is mixed with the solvent including water or both of water and binder. It is particularly preferred for the inorganic compound combined with the binder to be mixed in the hydrotalcite-type compounds dispersion (slurry).
After mixing the hydrotalcite-type compound and the binder, the mixture is preferably diluted with water and/or an alcohol. Dilution with water or alcohol after mixing can be used to adjust the transparency of the coating and the pH, hardness and volatility of the composition, thus allowing appropriate adjustment to match the properties and purpose of the substrate on which the coating is to be formed. Here, the solvent for dilution may be organic solvent.
For the means described above, the liquid preferably contains the hydrotalcite-type compound of 5 ppm to 1 wt %. By setting the hydrotalcite-type compound concentration within this range it is possible to obtain a coating with hydrophilic properties while minimizing aggregation of the liquid.
Embodiments of the invention will now be explained in detail. For the invention, attention was focused on substances represented by the following formula, which are known as hydrotalcite-type compounds.
[M2+1xe2x88x92xM3+x(OH)2]x+[Anxe2x88x92x/nmH2O]xxe2x88x92xe2x80x83xe2x80x83(1)
Here, M2+ is a bivalent metal ion such as Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+ or Zn2+, M3+ is a trivalent metal ion such as Al3+, Fe3+, Cr3+, Co3+ or In3+, and An- is an anion of valency n such as OHxe2x88x92, Fxe2x88x92, Clxe2x88x92, Brxe2x88x92, NO2xe2x88x92, CO32xe2x88x92, SO42xe2x88x92, Fe(CN)33xe2x88x92, CH3COOxe2x88x92, oxalate ion or salicylate ion. x is an value such that 0  less than xxe2x89xa60.33.
These hydrotalcite-type compounds have a layered structure with alternate stacking of a base layer represented by [M2+1xe2x88x92xM3+x(OH)2]x+ and an intermediate layer represented by [Anxe2x88x92x/nmH2O]xxe2x88x92. In the base layer, M2+ may be substituted with M3+ to a maximum molar ratio of M2+:M3+=2:1, and a larger value for x, indicating the degree of substitution, will result in a higher charge for the base layer; the charge is neutralized by Anxe2x88x92 in the intermediate layer, and the remaining space of the intermediate layer is filled with H2O (water of crystallization).
Hydrotalcite-type compounds have anion-exchange properties, and their other properties as antacids (neutralizing agents), aggregating agents, moisture absorbers, anion-absorbers, etc., are known in the prior art.
The present invention provides a film-forming composition by combination of the aforementioned hydrotalcite-type compound with a binder. Here, the hydrotalcite-type compound may be dissolved in water in the presence of an acid before its mixture with the binder. Once the hydrotalcite-type compound has been dissolved or dispersed in water and the binder mixed therewith, it may be further diluted with water or an alcohol.
The binder used may be an inorganic binder such as silica (silicic acid) or an organic binder such as an acrylate resin, and the use of colloidal silica in particular can give a film-forming composition with long-term stability since colloidal silica will readily bond with the hydrotalcite-type compound. Water or an alcohol, organic solvents, or a mixture thereof, may be used as a diluting solvent after preparation of the stock solution. However, it is preferred to use one or more types of alcohols selected from among alcohols such as methanol, ethanol, isopropyl alcohol and the like, to increase the stability and dryability of the coating solution.
Stable substances among hydrotalcite-type compounds include those having the collective chemical name of magnesium-aluminum-hydroxide-carbonate hydrates (Mg6Al2(OH)16CO34H2O) (hereunder referred to simply as xe2x80x9chydrotalcite-type compound Axe2x80x9d). Specifically, these include the powder product DHT-6 (trademark of Kyowa Chemical Industry Co., Ltd.) with a mean particle size of about 3 xcexcm; the polycrystalline particles Kyoward 100, Kyoward 200, Kyoward 300, Kyoward 400, Kyoward 500PL, Kyoward 500SH, Kyoward 500SN, Kyoward 1000 and Kyoward 2000 (all trademarks of Kyowa Chemical Industry Co., Ltd.) with a mean particle size of about 20-300 xcexcm, and the amorphous products Kyoward 600 and Kyoward 700 with a mean particle size of about 20-300 xcexcm. These forms of hydrotalcite-type compound A have anion substitutability so that the carbonic acid groups in it are substituted by other anions. The mixture of the hydrotalcite-type compound and binder may be carried out under neutral or acidic, but preferably acidic, conditions. When they are mixed under alkali conditions, the properties of the hydrotalcite-type compound promote aggregation, thus hampering the performance of the coating solution. An acidic liquid containing the binder may be used to control the acidity of the coating solution, or an acid may be added separately during mixing to maintain the acidity of it.
Primer A (trademark of Colcoat Co.) and Ceramate (trademark of shokubai kasei Co.) are products containing colloidal silica which is most preferred as the inorganic binder. Primer A is a silica dispersion containing colloidal silica and also containing primarily isopropyl alcohol as the solvent.